With the improvement of the technology, multifunctional image reading apparatus has been introduced. U.S. Pat. No. 4,879,604 discloses a scanning device with multiple functions, i.e. a reflected-light and a passing-light modes. It is known that, as the cover providing a white background for the reflected-light mode is replaced by a light source module for the passing-light mode, the multifunction image reading apparatus then can scan image on original of the type which light can pass through. The scan module within the image reading apparatus generally includes an image sensor device and a plurality of lenses. Image sensor device, or alternatively named as opto-electric converter, may be either well known CCD, or CIS, which receives the passing-light representative of the information on the original.
During the passing-light mode scanning process, the light source module for the passing-light mode moves in synchronization with the scan module along the original to be scanned and a fixed relative positional relationship between them are retained.
Generally, the scan module of the image reading apparatus will be reset or positioned at an initialization position upon power-on of the apparatus, or, is driven to the initialization position after completion of each pass of scanning process. Only when the scan module is positioned at the correct initialization position, the image data obtained are truly correct data corresponding to the correct portion of the original. Furthermore, under the passing-light mode, the light source module must also be initially positioned correctly; otherwise, the quality of the scan will be adversely affected. Accordingly, before the image reading apparatus conducts a passing-light mode scanning, the scan module is firstly positioned at a correct initialization position. Afterwards, the light source module is positioned to an initialization position. As well known in the arts, the conventional light source module includes a position sensor for positioning the light source module at an initialization position. Particularly, when the light source moves to a position which corresponds to and actuates the sensor, a signal from the position sensor is sent to a processor for instructing the scanning module to stop at the preset initialization position.
However, when implementing the above mentioned design, complexity with respect to design, manufacturing as well as assembly are involved. Furthermore, the yield rate drops due to its precision requirement involved during the assembly. If the position sensor and/or the actuator malfunctions during the scanning process, the overall result of such scanning process is considered as fail. Besides, more components in a design means higher cost.
Referring to FIG. 1a, the influence of distance of the light travels on the illumination value will be described. According to the well known optical theory, since distance DOA&gt;distance DOB&gt;distance DOC, when those points A, B and C are both exposed under the same light source, the illumination intensity on those three points is IA&lt;IB&lt;IC. Similarly, since distance DOE&gt;distance DOD&gt;distance DOC, the illumination intensity on points E, D and C is IE&lt;ID&lt;IC. In conclusion, point C has the brightest illumination intensity.
Referring to FIG. 1b, the influence of the incident angle to the illumination intensity will be further described. Since the .angle.AOC&gt;.angle.BOC&gt;.angle.COC, when the same light source emits a light beam from points A, B, and C, respectively, the illumination intensity at point O due to the light source at point C is larger than that at point O due to the light source at point B. And the illumination intensity at point O due to the light source at point B is larger than that at point O due to the light source at point A. Similarly, since .angle.EOC&gt;.angle.DOC &gt;.angle.COC, when the same light source emits a light beam from points E, D, and C, respectively, the illumination intensity at point O due to the light source at point C is larger than that at point O due to the light source at point D. And the illumination intensity at point O due to the light source at point D is larger than that at point O due to the light source at point E. In conclusion, point O has the brightest illumination intensity from the light source located on point C.
In light of the above observation, when there is no original or document on the document platen, the smaller of the light travel distance is and the smaller of the incident angle is, the brighter of the illumination intensity sensed by the scan module will be.
Based on the basic function of the image reading apparatus, the invention employs the principle recited above in positioning the light source module at its initialization position.